Seasonal Study: Fall Foliage

A

The transition of leaves in autumn is a captivating spectacle in numerous regions across North America. The hues are breathtaking, yet the query of why certain trees exhibit yellow or orange shades while others showcase red or purple tones has long intrigued researchers.

B

During summer, leaves are green due to abundant chlorophyll, a molecule that harnesses sunlight and converts it into essential resources for the tree. As autumn approaches in the northern hemisphere, solar energy diminishes significantly. For many trees—except evergreen conifers—the optimal strategy is to cease photosynthesis until spring. Instead of retaining now redundant leaves throughout winter, trees conserve resources by shedding them. Prior to leaf fall, the tree dismantles chlorophyll molecules, returning valuable nitrogen to the twigs. As chlorophyll diminishes, other colors suppressed by it during summer emerge. This revelation accounts for the autumnal yellows and oranges, though not the striking reds and purples observed in trees like maple or sumac.

C

The origin of red hues is well-established: anthocyanins, water-soluble plant pigments reflecting the red to blue spectrum of visible light. They are part of a class of sugar-based chemical compounds known as flavonoids. However, the production of anthocyanins in leaves concurrently with leaf shedding remains puzzling. Why would a tree invest in creating new chemicals in its leaves when it's already focused on withdrawing and preserving existing ones?

D

Various hypotheses concerning anthocyanins suggest they may serve as a chemical shield against insect or fungal attacks, or attract fruit-eating birds, or enhance a leaf's resistance to freezing. Nonetheless, each theory faces challenges, notably the short period during which leaves exhibit redness, making the energy expenditure for anthocyanin production outweigh potential anti-fungal or anti-herbivore benefits.

E

Another theory posits that trees produce vibrant red hues to signal to herbivorous insects their vitality and capability to deploy chemical defenses against infestation. If insects respond to these signals, they might choose less resilient hosts for egg-laying. However, this theory lacks empirical support regarding whether stronger trees indeed display brighter leaves or if insects select hosts based on color intensity.

F

The 'light screen' hypothesis emerges as the most plausible explanation for why leaves synthesize anthocyanins while preparing for winter. Paradoxically, this hypothesis suggests that red pigment production in autumn leaves shields chlorophyll, the light-absorbing compound, from excessive light. Why protect chlorophyll when it's naturally adept at light absorption? Why safeguard chlorophyll during breakdown for salvage?

G

Chlorophyll, although adept at capturing sunlight energy, can become overwhelmed, especially in drought, low temperature, or nutrient-deficient conditions. This oversensitivity intensifies in autumn when leaves dismantle their internal mechanisms. The absorbed energy isn't immediately utilized as in summer leaves, rendering fall leaves susceptible to oxygen's damaging effects from excited chlorophyll molecules.

H

Indicators hint at the processes behind red leaf coloration. Redder leaves often face more sunlight, especially on their upper surfaces. Intense red hues thrive under dry, sunny days and cool nights, mirroring conditions that make leaves vulnerable to excessive light. Trees, like maples, tend to exhibit deeper reds in colder, stressed environments with more sensitive chlorophyll requiring increased sun protection.

I

What remains not completely understood, however, is the reason some trees opt to create red pigments while others choose not to, instead displaying their orange or yellow tones. Do these trees possess alternative methods to avoid excessive light exposure in autumn? Their narrative, though less visually striking, will undoubtedly prove to be as nuanced and intricate.

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* photosynthesis: the generation of new substances from sunlight, water, and carbon dioxide.

Questions 14-18

Reading Passage 2 has nine paragraphs, A-l.

In which section can the subsequent details be located?

Write the correct letter, A-l, in boxes 14-18 on your answer sheet.

NB   You may use any letter more than once.

14   a description of the substance responsible for the red colouration of leaves

15   the reason why trees drop their leaves in autumn

16   some evidence to confirm a theory about the purpose of the red leaves

17   an explanation of the function of chlorophyll

18   a suggestion that the red colouration in leaves could serve as a warning signal

Queries 19-22

Fill in the notes below.

Choose ONE WORD ONLY from the passage for each answer.

Write your answers in boxes 19-22 on your answer sheet.

What's the rationale behind believing the 'light screen' theory?

●   The most vividly coloured red leaves are found on the side of the tree facing the 19…………….. .

●   The 20…………… surfaces of leaves contain the most red pigment.

●   Red leaves are most abundant when daytime weather conditions are 21……………. and sunny.

●   The intensity of the red colour of leaves increases as you go further 22……………..

Queries 10-12

Do the statements below align with the details in Reading Passage 2?

In boxes 23-25 on your answer sheet, write

TRUE           if the statement agrees with the information

FALSE          if the statement contradicts the information

NOT GIVEN    if there is no information on this

23   It is likely that the red pigments help to protect the leaf from freezing temperatures.

24   The ‘light screen’ hypothesis would initially seem to contradict what is known about chlorophyll.

25   Leaves which turn colours other than red are more likely to be damaged by sunlight.

Enquiry 26

Choose the correct letter A, B, C or D.

Write the correct letter in box 26 on your answer sheet.

Which of the following inquiries does the author explain?

A     why conifers remain green in winter

B     how leaves turn orange and yellow in autumn

C     how herbivorous insects choose which trees to lay their eggs in

D     why anthocyanins are restricted to certain trees

Answers:

14. C (Đoạn C, “The source of the red is widely known: it is created by anthocyanins, water-soluble plant pigments reflecting the red to blue range of the visible spectrum.”)

15. B (Đoạn B, “As fall approaches in the northern hemisphere, the amount of solar energy available declines considerably. …So rather than maintaining the now redundant leaves throughout the winter, the tree saves its precious resources and discards them.”)

16. H (Đoạn H, “Even if you had never suspected that this is what was going on when leaves turn red, there are clues out there.”)

17. B (Đoạn B, “Summer leaves are green because they are full of chlorophyll, the molecule that captures sunlight converts that energy into new building materials for the tree.”)

18. E (Đoạn E, “It has also been proposed that trees may produce vivid red colours to convince herbivorous insects that they are healthy and robust and would be easily able to mount chemical defences against infestation.”)

19. sun(light) (Đoạn H, “One is straightforward: on many trees, the leaves that are the reddest are those on the side of the tree which gets most sun.”)

20. Upper (Đoạn H, “Not only that, but the red is brighter on the upper side of the leaf.”)

21. Dry (Đoạn H, “t has also been recognised for decades that the best conditions for intense red colours are dry, sunny days…”)

22. North (Đoạn H, “And finally, trees such as maples usually get much redder the more north you travel in the northern hemisphere.”)

23. FALSE (Đoạn D, Some theories about anthocyanins have argued that they might act as a chemical defence against attacks by insects or fungi, or that they might attract fruit-eating birds or increase a leafs tolerance to freezing.”)

24. TRUE (Đoạn F, “Perhaps the most plausible suggestion as to why leaves would go to the trouble of making anthocyanins when they’re busy packing up for the winter is the theory known as the ‘light screen’ hypothesis.”)

25. NOT GIVEN (không có thông tin)